1. Field of the Invention
This present invention relates to a rudder for ships with a rudder blade which has a leading edge and a trailing edge. The rudder blade has two superimposed rudder blade sections, the leading edge sections and/or the trailing edge sections of which are offset in such a manner that the one leading edge section and/or the one trailing edge section is offset port or starboard and the other leading edge section and/or trailing edge section starboard or port and that the one leading edge and/or trailing edge section has a port-sided offset surface which projects over the other leading edge section and/or the other trailing edge section and the other leading edge section and/or trailing edge section has a starboard-sided offset surface which protrudes over the one leading edge section and/or trailing edge section.
2. Description of the Related Art
Such rudders are known in the prior art and are often designated as twisted rudders. Generally, for such rudders, the rudder blade is divided into an upper and a lower half or an upper and a lower rudder blade section along a section plane which is oriented normally substantially horizontally for a mounted rudder. For a few embodiments, for example for twisted rudders with horn, the separating line between the two rudder blade sections can also be configured in a profile view non linearly, for example graded. Both rudder sections are placed adjacent to each other and can be fixedly connected with each other. Each rudder blade section has a leading edge section and a trailing edge section. The front leading edge areas (or sections) of both rudder sections are offset relative to each other or are placed twisted relative to each other, whereas both side wall surfaces of the respective rudder blade sections converge into a single continuous trailing edge section. The offset or the twisting of the rudder blade is provided for these embodiments only in the front area which is turned to the propeller. Moreover, multiple twisted rudders are known for which the leading edge is divided into three sections or more, wherein a section is placed respectively offset to its adjacent sections. Furthermore, there are also known embodiments for which the trailing edge sections of the single rudder blade sections turned to the propeller are placed offset to each other. On the other side, the opposed leading edge sections turned to the propeller merge for this embodiment into a continuous single strip. Furthermore, embodiments are also possible for which the rudder blade sections of the leading edge as well as of the trailing edge are offset to each other, wherein for this embodiment typically the nose and trailing edge of a rudder blade section are offset to different sides, i.e. the one strip to the port side and the other side to the starboard side.
When mounted in a ship, the rudder blade is assigned to a propeller placed on a drivable propeller axis and connected with the hull of the ship, wherein the rudder blade is placed behind the propeller in the direction of motion of the ship and the rudder blade is placed in such a manner that the (front) nose edge is turned to the propeller and that the (rear) strip is turned away from the propeller. Moreover, the rudder comprises normally additionally to the rudder blade a rudder port for the rudder post and a rudder post.
The indication that the rudder blade sections are placed superimposed refers to the mounted state of the rudder blade in which usually a section is placed above the other. Generally speaking, both rudder blade sections are thus placed adjacent to each other. Due to the offset arrangement of the leading edges to each other, an offset surface which protrudes or projects normally laterally respectively beyond the other leading edge is created on each leading edge in the area in which both leading edges are adjacent to each other. Thus, there is a (90°) edge in the transition area between the two leading edges on each side which runs into one of the offset surfaces. A further (90°) edge is created on the inner side of the offset surfaces.
FIGS. 7 and 8 show examples of twisted rudders known from the prior art with leading edges (sections) offset to each other. The rudder blade 100 has respectively two superimposed rudder blade sections 10, 30, wherein the leading edge sections 11, 21 are offset in such a manner that the one leading edge (or leading edge section) 11 is offset to the port side BB and the other leading edge (or leading edge section) 21 to the starboard side SB. Both side wall surfaces 100a, 100b of the rudder blade 100 or of both rudder blade sections 10, 20 merge into a single continuous trailing edge 30. Since both leading edges 11, 21 are to be placed offset to each other for the twisted rudder, one leading edge has always to be respectively offset to the port side and the other to the starboard side. Due to the offset arrangement, there results respectively an offset surface 18 in the area of the transition between the leading edges 11, 21 on each rudder blade side. The offset surface 18 represented in FIG. 8 is formed by the part of the lower side of the upper leading edge 11 which projects over the lower leading edge 21. The offset surface existing on the opposite side (which is not depicted here) is formed correspondingly by the part of the upper side of the lower leading edge 21 which projects over the upper leading edge 11.
FIG. 9 shows a further example of a twisted rudder known from the prior art for which both rudder blade sections 10, 20 of the rudder blade 100 are offset to each other in the area of their trailing edge sections 30a, 30b. On the other hand, the leading edge 11 turned to the propeller in mounted state is configured continuous. Due to the offset arrangement, there results for this embodiment an offset surface 18 on each rudder side, wherein the offset surfaces 18 are configured between the transitions of the trailing edges (trailing edge sections 30a, 30b). The offset surface 18 depicted in FIG. 9 is formed by the part of the upper side of the lower trailing edge 30b which projects laterally over the upper trailing edge 30a. 
The advantage of such twisted rudders with two mirror-inverted cross section profiles consists on the one hand in avoiding the vapour bubble formation and on the other hand in avoiding erosion phenomena on the rudder which appear due to cavitation formation for high-speed ships with high loaded propellers. Moreover, the special configuration of the rudder blade contributes to a reduction of the fuel consumption. Besides a considerable protection against cavitation, there is thus also an improvement of the degree of efficiency. Furthermore, a considerable saving of weight is achieved. In particular, these improvements can be produced in that, due to the offset arrangement of the leading edges of both rudder blade sections, an adaptation to the twist in the propeller jet takes place.
For such rudders, because of the offset arrangement of the front leading edges or of the rear trailing edges and the thus caused angular transitions between the strips of the single rudder blade sections, it can come to a swirl of the current so that a.o. the risk of cavitation is increased. Moreover, in spite of the orientation of the single front leading edges or the rear trailing edges with respect to the twist of the propeller jet, in particular in the transition area between the strips, it can come to a separation of flow.
Furthermore, it is known in the prior the art to provide Costa bulbs on rudders. Costa bulbs are relatively big bulb or Zeppelin shaped bodies which are provided on rudder blades. Costa bulbs are fundamentally known and are sometimes also designated as propulsion bulbs. They are provided in prolongation of the propeller (shaft) axis in the area of the rudder blade and clearly protrude from the rudder blade in direction of the propeller and beyond the rudder blade.
In particular, Costa bulbs project so far from the rudder blade that they (nearly) come to rest on the hub of the propeller. The distance between the Costa bulb and the propeller or the propeller hub should generally be the lowest possible so that as much of the water flow as possible produced by the propeller flows on the outside along the Costa bulb and not between the Costa bulb and the propeller hub.
Due to this prolongation of the whole profile of the hub, it is achieved that only a light swirl of the water flowing off develops. This being, it is however disadvantageous that the Costa bulb exerts a strong influence onto the propulsion behavior of the ship. If it is provided on an existing rudder, the propulsion behavior is influenced negatively and must be specially adapted to the propulsion behavior of the ship which causes complicated and expensive tests and trials. If such an adaptation does not take place, due to the provision of the Costa bulb, the fuel consumption of the ship is considerably increased.